1. Field of the Invention
The present invention relates to a sand pile driving method for driving piles made of sand and the like in a ground for ground improvement.
2. Description of the Related Art
As a method to improve soft soil and the like, a sand compaction pile method (SCP method) for ground improvement, by which sand piles are driven in places in an area to be improved is conventionally known. Description will be given on the conventional sand pile driving method by the sand compaction pile method.
As shown in FIG. 1, a sand pile driving apparatus 1 includes a casing pipe 2 which is disposed in the vertical direction toward a main body of an unillustrated construction machine; a vibrator 3 which vibrates the casing pipe 2; a compaction member 4 which is located at the lower end of the casing pipe 2; and a piston cylinder mechanism 5 which enables the compaction member 4 to perform a reciprocating motion in the vertical direction.
Sand pile driving work by use of the sand pile driving apparatus 1 will be described. By the operation of the vibrator 3, the casing pipe 2 is penetrated into a ground 6 to a predetermined depth. Then, while reciprocating the piston cylinder mechanism 5, sand is discharged from the lower end of the casing pipe 2 and the discharged sand is compacted. At the same time, a pull out procedure to pull out the casing pipe 2 to a predetermined length is performed. By this pull out procedure, sand is filled into a space within the ground 6 which is made after the casing pipe 2 is pulled out.
the reciprocating operation is stopped and sand is resupplied into the casing pipe 2. Then, a procedure to pull out the casing pipe 2 upward is performed again. In this pull out procedure, sand is discharged and compacted while the piston cylinder mechanism 5 performs a reciprocating action. Hereafter, by performing compaction during the pull out procedure of the casing pipe 2 until the lower end thereof reaches the ground surface, a sand pile 7 as shown in FIG. 2A is driven in the space where the casing pipe 2 was penetrated. Such sand piles 7 are driven at adequate intervals.
In general, the strength of the actual ground 6 to be improved is not uniform and scattered in distribution. Therefore, a method that enables changing of one or both of the diameter and strength of the sand pile 7 in accordance to the strength of the ground 6 has been proposed by the inventor.
The prior proposals by the inventor are disclosed in the Japanese Patent Number 136138 and Number 1521542. For example, in a case where only the diameter of the sand pile 7 is changed, thrust force of the piston cylinder mechanism 5 which presses the sand pile 7 downward in the process of compaction is detected and the sand pile 7 is pressed until the thrust force reaches a given preset value. At an area of soft soil of the actual ground 6 in a depth, the sand pile 7 is subjected to compressive deformation in the direction of enlarging the diameter. Then, the thrust force reaches a given preset value and a sand pile 7 with a large diameter is driven.
At a hard ground area of the actual ground 6 in a depth, the thrust force reaches a given preset value before the diameter of the sand pile 7 is not enlarged enough. Thus, sand pile 7 which has a relatively small diameter is driven. In this way, uniform ground improvement, by reinforcing the ground in accordance with the softness of the actual ground, is realized by making sand piles 7 which are pressed by constant thrust force of the piston cylinder mechanism 5 in the process of compaction.